While the demand for increase in capacity and speed of data processing is increasing, a higher-density optical signal processing circuit is being requested.
While a planar optical circuit (PLC; Planar Lightwave Circuit) for which a silica glass-based material is used is conventionally used, in recent years, research and development of a technology for forming an optical circuit on an SOI (Silicon on Insulator) substrate by practically applying a silicon (Si) wire waveguide processing technology is proceeding.
In the technology for forming an optical circuit on an SOI substrate, an Si core is frequently covered with a SiO2 cladding layer to form an optical waveguide, and since the difference in refractive index between Si and SiO2 is great, light can be confined in a higher efficiency in the Si core. Therefore, an optical waveguide array having a small distance or a curved optical waveguide having low optical loss can be formed, and optical waveguides can be formed in a high density.
Further, in order to avoid crossing of optical waveguides in a plane to reduce the loss, also a three-dimensional cross waveguide that implements a three-dimensional cross has been proposed. In the three-dimensional cross waveguide, one core having a taper region, which has a size that decreases toward a tip end thereof, is provided on a certain plane, and another core having a taper region, which has a size that decreases toward a tip end thereof, is provided on another plane having a distance from a substrate different from that of the one plane such that a tip end thereof opposes to that of the taper region of the one core so that propagation light is shifted in the heightwise direction.